Parallel recording and reading of diffractive memory using multiple object beams

ABSTRACT

A parallel recording of diffractive storage system is provided. The recorded information is cellular for its designation. A laser beam is data modulated through a single reference beam and a plurality of object beams positioned at different angles. These object beams intersect the reference beam within a recording medium to form a plurality of data loaded pattern simultaneously.

FIELD OF THE INVENTION

The present invention generally relates to photonics data memorydevices. In particular, the present invention relates to fast access tophotonics data memory devices using multiple object beams.

BACKGROUND OF THE INVENTION

There is a strong interest in high-capacity data storage systems withfast data access due to an ever-increasing demand for data storage.Limitations in the storage density of conventional magnetic memorydevices have led to considerable research in the field of opticalmemories. Holographic memories have been proposed to supersede theoptical disc (compact disc read only memories, or CD-ROMs, and digitalvideo data, or DVDs) as a high-capacity digital storage medium. The highdensity and speed of holographic memory results from the use ofthree-dimensional recording and from the ability to simultaneously readout an entire page of data. The principal advantages of holographicmemory are a higher information density, a short random-access time, anda high information transmission rate.

In holographic recording, a light beam from a coherent monochromaticsource (e.g., a laser) is split into a reference beam and an objectbeam. The object beam is passed through a spatial light modulator (SLM)and then into a storage medium. The SLM forms a matrix of cells thatmodulate light intensity with grey levels. The SLM forms a matrix ofshutters that represents a page of binary or grey-level data. The objectbeam passes through the SLM, which acts to modulate the object beam withbinary information being displayed on the SLM. The modulated object beamis directed to one point, after an appropriate beam processing, where itintersects with the reference beam after being routed by an addressingmechanism. It is also contemplated that for polychromatic holography,the polychromatic hologram may be recorded with more than one wavelengthfrom different lasers or from the same multiline laser at the same time.In other words, the recording can be operating with several wavelengthsin the holographic multiplexing process.

An optical system consisting of lenses and mirrors is used to preciselydirect the optical beam encoded with the packet of data to theparticular addressed area of the storage medium. Optimum use of thecapacity of a thick storage medium is realized by spatial and angularmultiplexing that can be enhanced by adding frequency polarization,phase multiplexing, etc. In spatial multiplexing, a set of packets isstored in the storage medium and shaped into a plane as an array ofspatially separated and regularly arranged subholograms by varying thebeam direction in the X-axis and Y-axis of the plane. Each subhologramis formed at a point in the storage medium with the rectangularcoordinates representing the respective packet address as recorded inthe storage medium. In angular multiplexing, recording is carried out bykeeping the X- and Y-coordinates the same while changing the irradiationangle of the reference beam in the storage medium. By repeatedlyincrementing the irradiation angle, a plurality of packets ofinformation is recorded as a set of subholograms at the same X- andY-spatial location.

A volume (thick) hologram requires a thick storage medium, typically athree-dimensional body made up of a material sensitive to a spatialdistribution of light energy produced by interference of a coherentlight beam and a reference light beam. A hologram may be recorded in amedium as a variation of absorption or phase or both. The storagematerial responds to incident light modulation patterns causing a changein its optical properties. In a volume hologram, a large number ofpackets of data can be superimposed, so that every packet of data can bereconstructed without distortion. A volume (thick) hologram may beregarded as a superposition of three-dimensional gratings recorded inthe depth of the emulsion, each satisfying the Bragg law (i.e., a volumephase grating). The grating planes in a volume hologram produce changesin refraction and/or absorption.

While holographic storage systems have not yet replaced current compactdisc (CD) and digital video data (DVD) systems, many advances continueto be made which further increase the potential of storage capacity ofholographic memories. This includes the use of various multiplexingtechniques such as angle, wavelength, phase-code, fractal, peristrophic,and shift. However, methods for recording information in highlymultiplexed volume holographic elements, and for reading them out, havenot proved satisfactory in terms of throughput, crosstalk, and capacity.

Currently, one object beam and multiplexing (i.e., angular multiplexing)of a reference beam are used in holographic memory recording. Therefore,to access one packet of data, it is necessary to record and read with aspecific reference angle. This is time-consuming, since the recording isdone sequentially for all angles of the reference beam.

BRIEF DESCRIPTION OF THE DRAWINGS

In order to facilitate a fuller understanding of the present invention,reference is now made to the appended drawings. These drawings shouldnot be construed as limiting the present invention, but are intended tobe exemplary only.

FIGS. 1A and 1B are schematic representations of an apparatus forrecording multiple holograms in accordance with one embodiment of theinvention.

FIG. 2A is a schematic representation of the beam splitting device usinga cascade of beam splitters in accordance with one embodiment of theinvention.

FIG. 2B is a schematic representation of a beam splitting device using adiffractive optic element in accordance with one embodiment of theinvention.

FIG. 3A is a schematic representation of the apparatus shown in FIG. 1for recording multiple holograms in accordance with one embodiment ofthe invention.

FIG. 3B is a schematic representation of a recording system using a DOEas splitter in accordance with one embodiment of the invention.

FIG. 4 is a schematic representation of the apparatus for readingmultiple holograms in accordance with one embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention introduces the use of one reference beam andseveral object beams for recording and reading holographic memory. Theuse of multiple object beams allows parallel recording and reading at ahigh speed. The parallel recording and reading are achieved by movingand scanning a recording medium respectively, using one reference beam.The addressing of one point allows one to read a plurality of packetslocated in that one point. The use of multiple object beams in therecording system reduces the recording time by a factor of the angularmultiplexing number (of the reference beam). Furthermore, it wouldeliminate the use of rotating mirrors to angular multiplexing of thereference beam. By the same token, the access time in reading data fromthe recorded memory is also reduced by a factor of the angularmultiplexing number. Furthermore, it may be that just one acousto opticassociated to amplification grating is needed to read all matrices, andthe rotating actuator in a traditional reading system may be eliminated.

Further advantages and novel features of the present invention willbecome apparent to those skilled in the art from this disclosure,including the following detailed description, as well as by practice ofthe invention. While the invention is described below with reference toillustrative embodiments, this description is not intended to beconstrued in a limiting sense. Those of ordinary skill in the art havingaccess to the teachings herein will recognize additionalimplementations, modifications, and embodiments, as well as other fieldsof use, which are within the scope of the invention as disclosed andclaimed herein, and with respect to which the invention could be ofsignificant utility.

Storing/Recording Phase

Referring to FIGS. 1A and 1B, there are shown schematic representationsof an apparatus for holographic recording using one reference beam and aplurality of object beams. The system 100A includes a laser 101, a beamsplitter system 102, a recording medium 103, a plurality of displays 104₁ to 104 _(N) (where N is a positive integer), and a plurality of lenses105 ₁ to 105 _(N).

The laser 101 provides a laser beam 106 (i.e., coherent light beam) tothe beam splitter system 102. In one embodiment, the laser 101 is a YAGdoubled laser, i.e., a solid state laser. A rod of YAG material emitslaser light in the infrared to the laser 101. The laser light associateswith a nonlinear crystal that produces double the YAG-emitted lightfrequency. The laser beam 106 emanating from laser 101 is split into areference beam 110 and a main object beam 107 (shown in FIGS. 2A and 2B)by the sub-splitter 111, which are then split into a plurality of objectbeams 120 ₁ to 120 _(N). The main object beam 107 is split into aplurality of object beams 120 ₁ to 120 _(N) by a multiple beam splitter109. The multiple beam splitter 109 may be internal or external to thebeam splitter system 102. In one embodiment, the multiple beam splitter109 is a diffractive optic element (DOE) and in another embodiment themultiple beam splitter 109 includes a cascade of beam splitters 130 ₁ to130 _(N-1), as shown in FIG. 2A, and a mirror 131.

Referring to FIG. 2A, there is shown a schematic representation of amultiple beam splitter 109 as a cascade of beam splitters according toone embodiment of the invention. The beam device 109 includes aplurality of cascade beam splitters 130 ₁ to 130 _(N-1) and the mirror131. The main object beam 109, split from the laser beam 107, is thensplit into the plurality of object beams 120 ₁ to 120 _(N) by themultiple beam splitter 109 (i.e., cascade of beam splitters and mirror).The beam splitting power is controlled to have the same beam intensitiesrouted to the SLMs set. The light-split intensity depends on thereflectivity of the coated layer on the splitter substrate. In oneembodiment, every object beam has the same photonic power so that everySLM can receive approximately the same light power. In other words, thisprovides basically the same splitted power to each beam addressing theset of SLMs. Every recorded plate recording substrate with the sameintensity on each of the charge-coupled devices (CCDs), every recordedplate provides a set of packets with the same energy due to thesame-recorded effect on the recording substrate.

As described above, the splitting may be done by means of diffractivegrating or by a cascade of beam splitters. The high-density main objectbeam 107 is equally split on every SLM. This splitting provides everySLM with an equal light power. After the data is loaded by every SLM oneach split laser beam, they are focused by the set of lenses 105 ₁ to105 _(N) onto the same point on the recorded substrate of the recordingmedium 103.

In the case where the energy level of each object beam does not matchone another, adjusting the sensitivity of each SLM device is possible.Furthermore, if the energy level of each beam is different, opticalattenuators may be selected to adapt jointly to the energy on the beamand the sensitivity of the SLMs. One SLM in the plurality of SLMs is a“master” SLM for the other SLMs. The light power is adjusted on thereference of this master light by using attenuators. Every attenuator isadapted for different beams. The optical density of the attenuator isadapted to the power available on the SLMs. For example, if one SLMreceives energy differently from its neighbor SLM, an attenuator is usedto adjust the energy on the neighbor SLM so that the intensity of theimage supporting the recorded packet is the same. This provides uniformintensity of the packet of data read.

Referring to FIG. 2B, there is shown a schematic representation of thediffractive optical element (DOE) as the multiple beam splitter 109according to one embodiment of the invention. The main object beam 107is split from the laser beam 106 and is emitted to the DOE 109. The DOE109 produces a multitude of diffracted beams (i.e., object beams 120 ₁to 120 _(N)). These diffractive beams are routed to arranged mirrors(not shown) and to the SLMs set individually.

The recording medium 103 has a plurality of cells to hold the recordedinformation. The recording medium 103 is a plate holographic memorydevice that contains information stored during a phase of storinginformation. The recording (i.e., storage) medium is typically athree-dimensional body made up of a material sensitive to a spatialdistribution of light energy produced by interference of the objectbeams 120 ₁ to 120 _(N) and the reference light beam 110. A hologram maybe recorded in a medium as a variation of absorption or phase or both.The storage material responds to incident light modulated patternscausing the change in its optical properties. In a volume (thick)hologram, a large number of packets of data can be superimposed, so thatevery packet of data can be reconstructed without distortion. A volumehologram may be regarded as a superposition of three-dimensionalgratings recorded in the depth of the layer of the recording material,each satisfying the Bragg law (i.e., a volume phase grating). Thegrating modulation in a volume hologram produces change in refractionand/or absorption. The recording medium 103 may be arranged in the formof a flat layer, herein referred to as a matrix. Each of a plurality ofpoints on the matrix is defined by its rectilinear coordinates (X,Y). Apoint in physical space, defined by its rectilinear coordinates,contains a plurality of packets.

In one embodiment, the recording medium 103 is constructed of organicmaterial, such as a polypeptide material, and made in accordance withthe techniques described in the co-pending patent application entitled“Photonics Data Storage System Using a Polypeptide Material and Methodfor Making Same,” Serial No. PCT/FR01/02386, which is hereinincorporated by reference.

The plurality of object beams 120 ₁ to 120 _(N), pass through theplurality of displays (e.g., SLMs) 104 ₁ to 104 _(N) and then into thestorage medium 103. In other words, each object beam is loaded with databy its corresponding display, and that display is illuminated by themain object beam 107 after splitting by the multiple beam splitter 109.The object beams 120 ₁ to 120 _(N), are positioned at different angles.Each display of the plurality of displays 104 ₁ to 104 _(N) forms amatrix of shutters that represents a packet of binary data. The objectbeams 120 ₁ to 120 _(N) may be filtered and collimated. The object beams120 ₁ to 120 _(N) are directed to the displays 104 ₁ to 104 _(N), whichdisplay images to be recorded. The object beams 120 ₁ to 120 _(N) becomemodulated by the information to be recorded by means of reflection offor transmission through the displays 104 ₁ to 104 _(N).

The displays 104 ₁ to 104 _(N) may be any devices for displaying datapackets in a system, such as spatial light modulators (SLMs) or liquidcrystal light valves (LCLVs). In one embodiment, the plurality of bitsrepresented on the display screen of the displays 104 ₁ to 104 _(N) ispresented as a two-dimensional pattern of transparent and opaque pixels(i.e., data packet). The data packet displayed is derived from anysource such as a computer program, the Internet, and so forth. In anInternet storage application, the packets displayed may be formattedsimilarly to the packets of the Internet.

The reference laser beam 110 interferes coherently with the object beams120 ₁ to 120 _(N) to form the interference patterns or holograms, whichare stored in the recording medium 103 due to the perturbation in therefractive index. Thus, each hologram is stored at a unique angle of theobject beams 120 ₁ to 120 _(N). The separation between the variousholograms stored within the same volume relies on the cross talkseparation and angular tolerancing, in order to allow its retrieval onlyfor a defined angle value.

As stated above, the object beams 120 ₁ to 120 _(N) pass through thedisplays 104 ₁ to 104 _(N), which act to modulate the object beams 120 ₁to 120 _(N) with the binary information. The object beams 120 ₁ to 120_(N) are then directed to a defined point on the recording medium 103where they intersect with the reference beam to create a plurality ofholograms representing packets of data. The plurality of lenses 105 ₁ to105 _(N) is used to converge the modulated object beams 120 ₁ to 120_(N) and to focus the beams to the recording medium 103. In other words,the modulated beams 120 ₁ to 120 _(N) become reduced by means ofsuitable lenses 105 ₁ to 105 _(N) so that the point of convergence ofthe modulated object beams 120 ₁ to 120 _(N) lies slightly beyond therecording medium 103. The object beams 120 ₁ to 120 _(N) are positionedat different angles so that plurality of data packets is recorded at onepoint of the recording medium 103. The different angle is calculated oradjusted to a position to avoid crosstalk between two output neighboringbeams.

Referring to FIG. 3A, there is shown a more detailed schematicrepresentation of the apparatus shown in FIG. 1 for holographicrecording using multiple object beams. The system 300 includes thesingle reference beam 110, the set of several object beams 120, to 120_(N), and the recording medium 103. The reference beam 110 and theobject beams 120 ₁ to 120 _(N) intersect to form patterns to be recordedon the recording medium 103 at an X,Y location. In other words, for oneX,Y location, the recording is done by one reference beam and severalobject beams intersect. Each object beam is oriented according to aspecific angle so that it can be focused on one point in the recordingmedium 103. The object beams 120 ₁ to 120 _(N) are positioned atdifferent angles, similar to the way the reference beam is angularlymultiplexed in the previous recording system using angular multiplexingfor the reference beam. All the multiplexed object beams 120 ₁ to 120_(N) are recorded simultaneously at one point by using one referencebeam 110. To go to the next recording point, the recording medium ismoved in X, Y direction by a moving system (not shown). This is thespatial multiplexing that is carried out by sequentially changing therectilinear coordinates. The plurality of object beams 120 ₁ to 120 _(N)focus on the recording medium 103 so that N number of separate imagesare recorded at a unique position in a plane defined by its coordinates(X,Y).

Referring to FIG. 3B, there is shown a recording system 300B using a DOEas splitter to generate multiple diffractive beams in accordance withone embodiment of the invention. The system 300 includes the lasersource 101, the beam splitter system 102, the DOE 109, a plurality ofmirrors 301 ₁ to 301 _(N), a plurality of expanding and collimating beamdevices 305 ₁ to 305 _(N), the plurality of displays 104 ₁ to 104 _(N),the plurality of focusing lenses 105 ₁ to 105 _(N), and the recordingplate 103.

The functions and characteristics of the laser source 101, the beamsplitter 102, the displays 104 ₁ to 104 _(N), the focusing lenses 105 ₁to 105 _(N), and the recording plate 103 are already described above.The DOE 109 produces a plurality of multitude of diffracted beams 304 ₁to 304 _(N). The plurality of mirrors 301 ₁ to 301 _(N) directs thebeams to the plurality of corresponding expanding and collimating beamdevices 305 ₁ to 305 _(N) for expanding and collimating the multiplediffracted beams. The expanded and collimated beams pass through thedisplays 104 ₁ to 104 _(N) to become a plurality of object beams whichintersect with the reference 110 at a point on the recording plate 103to form a plurality of data packets.

Reading Phase

Referring to FIG. 4, there is shown a schematic representation of anapparatus for holographic reading using one reference beam to read aplurality of packets simultaneously. The system 400 includes a laser 401(e.g., laser 101), the recording medium 103, a plurality of imagesensors (i.e., phototransistor devices, CCDs) 405 ₁ to 405 _(N), and aplurality of lenses 406 ₁ to 406 _(N). The laser 401 emits a coherentlight beam 410 (i.e., reading/reference beam) to the recording medium103. The reference beam 410 may be the same as the reference beam 110.The recording medium 103 has a plurality of cells that hold the recordedinformation

Retrieving the recorded/stored information from the diffractive memorydevice 103 requires the use of the reference beam 410 (i.e., read beam)whose characteristics correspond to those employed for writing (i.e.,reference beam 110) or for storage. The reference beam 410 inducesdiffraction in the polypeptide layer due to perturbation in therefractive index corresponding to the characteristics of the beamsinterference, thereby creating a plurality of stored packets related tomodulated beams 430 ₁ to 430 _(N).

The reference beam 410 is positioned in order to access a plurality ofdata packets contained at a defined point (X,Y) on the matrix in therecording medium 103. The reading procedure is similar to the writing orrecording procedure. However, the reading procedure may be carried outwith a greater degree of tolerance than the recording procedure. It ispossible to use a very compact laser source of a solid-state type forthe reading process because the laser power necessary for reading ismuch lower than the one for recording.

The plurality of data packets in the diffractive memory device 103 arereconstructed simultaneously by shining the reference beam 410 at thesame location in which the data packets were recorded. The referencebeam 410 diffracted by the diffractive memory device 103 forms thereconstruction, which is detected by the plurality of arrays of imagesensors 405 ₁ to 405 _(N). The reference beam 410 is configured toaddress the plurality of packets at different locations in thediffractive memory device 103. The plurality of lenses 406 ₁ to 406 _(N)are positioned at different angles to focus output beams produced by thediffraction of the reference beam 410 onto the image sensors 405 ₁ to405 _(N). In one embodiment, the reference beam 410 is positionedperpendicular to the recording plate for recording. Therefore, there isno angular deflection in parallel reading. The read beam is diffractedfor the reading into plurality of channels simultaneously. Every channelis materialized by an output beam loaded with a data packet that ispositioned with a programmed angular value. This angular value is fittedto angular positioning of the beam used to record, data from thedisplays 104 ₁ to 104 _(N). The X,Y scanning is performed to proceedfrom one point to the next for the next reading of information on thediffractive memory device 103 diffracted and from there the outputpackets are focused.

The laser reference beam (i.e., reading beam) 410 is shaped and directedonto the recorded medium 103 and from there focused by imaging lenses406 ₁ to 406 _(N) onto image sensors (e.g., CCD cameras) 405 ₁ to 405_(N), each of which has a number of pixels adapted to the desiredresolution. The digital output of the image sensors 405 ₁ to 405 _(N) isfurther processed by a computer (not shown).

The reference beam 410 (i.e., read beam) emanates from the low-powerlaser 401. Typically the reference beam is less than 5 mW. The laser 401may be a helium-neon or semiconductor-type laser. The reference beam 410may be modulated by means of one or more transformation activators (notshown) lying in the optical path of the beam for reading data recordedin a plurality of points.

The present invention is not to be limited in scope by the specificembodiments described herein. Indeed, this application is intended tocover any modifications of the present invention, in addition to thosedescribed herein, and the present invention is not confined to thedetails which have been set forth. Thus, the scope of the inventionshould be determined by the appended claims and their legal equivalents,rather than by the examples given.

1. An apparatus comprising: a recording medium for recording informationdesignated to a cell; a laser for transmitting a single reference beamto the recording medium; and a multiple beam splitter for transmitting aplurality of object beams to the recording medium at different angles,the object beams intersecting the reference beam within the recordingmedium to form a plurality of patterns.
 2. The apparatus according toclaim 1 further comprising a plurality of displays for displaying theinformation to be recorded, the object beams being modulated byreflection off or transmission through the displays.
 3. The apparatusaccording to claim 2 further comprising a plurality of lenses forconverging the modulated object beams and focusing the modulated beamsto a point on the recording medium.
 4. The apparatus according to claim1 wherein the recording medium is a holographic plate memory device. 5.The apparatus according to claim 2 wherein the displays are spatiallight modulators (SLMs).
 6. The apparatus according to claim 1 whereinthe cell includes the plurality of interference patterns.
 7. Theapparatus according to claim 1 wherein the recording medium is made ofpolypeptide material.
 8. An apparatus comprising: a recording medium forrecording information designated to a cell; a laser for transmitting asingle reference beam to the recording medium; and a multiple beamsplitter for transmitting a plurality of object beams to the recordingmedium at different angles, the object beams simultaneously intersectingthe reference beam within the recording medium to form a plurality ofpatterns.
 9. An apparatus comprising: a recording medium for recordinginformation designated to a cell; a laser for transmitting a singlereference beam to the recording medium; a multiple beam splitter fortransmitting a plurality of object beams to the recording medium atdifferent angles, the object beams intersecting the reference beamwithin the recording medium to form a plurality of patterns; and aplurality of displays for displaying the information to be recorded, theobject beams being modulated by reflection off or transmission throughthe displays.
 10. An apparatus comprising: a recording medium forrecording information designated to a cell; a beam splitter system fortransmitting a single reference beam to the recording medium; anmultiple beam splitter for simultaneously transmitting a plurality ofobject beams to the recording medium at different angles, the objectbeams intersecting the reference beam within the recording medium toform a plurality of patterns; and a plurality of displays for displayingthe information to be recorded, the object beams beg modulated byreflection off or transmission through the displays.
 11. A methodcomprising: transmitting a single reference beam to a recording medium;positioning a plurality of object beams at different angles;transmitting a plurality of object beams to the recording medium; andintersecting the reference beam and the plurality of object beams withina cell in the recording medium to form a plurality of patterns.
 12. Themethod according to claim 11 further comprising: displaying informationto be recorded on a plurality of displays; and modulating the objectbeams by reflecting off or transmitting the information through thedisplays.
 13. The method according to claim 12 further comprisingconverging the modulated object beams.
 14. The method according to claim13 wherein the recording medium is a holographic plate memory device.15. The method according to claim 12 wherein the displays are spatiallight modulators.
 16. The method according to claim 11 wherein the dataloaded on the object beams are stored through the plurality ofinterference patterns.
 17. The method according to claim 11 wherein therecording medium is made of polypeptide material.
 18. A methodcomprising: transmitting a single reference beam to a recording medium;arranging a plurality of object beams at different angles;simultaneously transmitting a plurality of object beams to the medium;and intersecting the reference beam and the plurality of object beamswithin a cell in the recording medium to form a plurality of patterns.19. A method comprising: transmitting a single reference beam to arecording medium; arranging a plurality of object beams at differentangles; transmitting a plurality of object beams to the medium;intersecting the reference beam and the plurality of object beams withina cell in the recording medium to form a plurality of patterns; anddisplaying information to be recorded on a plurality of displays. 20.The method according to claim 19 further comprising modulating theobject beams by reflecting off or transmitting the information throughthe displays.
 21. The method according to claim 20 further comprisingconverging the modulated object beams.
 22. An apparatus comprising: amemory device including a cell for containing recorded information; abeam splitter system for transmitting a single reference beam to thememory device to read the recorded information; and a multiple beamsplitter for transmitting a plurality of object beams to the recordingmedium at different angles, the object beams intersecting the referencebeam within the recording medium to form a plurality of patterns. 23.The apparatus of claim 22 wherein the multiple beam splitter is adiffractive optic element.
 24. The apparatus of claim 22 wherein themultiple beam splitter is a cascade of beam splitters.
 25. An apparatuscomprising: a photonics data memory device having a plurality ofpatterns; a beam splitter system for transmitting a single referencebeam to the memory device to read output data packets stored through theplurality of patterns; a plurality of lenses for forming images producedby diffraction of the patterns; and means for converting the images intoelectrical signals.
 26. The apparatus according to claim 25 whereinmeans for converting are a plurality of CCDs (charge-coupled devices).27. The apparatus according to claim 25 further comprising a computerfor processing and analyzing the electrical signals.